The present invention relates generally to television tuning systems and more particularly to so-called "direct-access" or "signal seeking" tuning systems.
In direct-access type tuning systems, a viewer is typically provided with a keyboard by which he can command the receiver to tune itself to a particular channel. The tuning system is programmed to "know" what its tuner's correct local oscillator frequency should be for the selected channel and develops a tuning voltage that will put the oscillator at the correct frequency.
In the process of tuning the oscillator, the oscillator frequency is constantly sensed by a frequency monitoring circuit which divides the oscillator frequency down to a lower frequency, counts the lower frequency, and compares it to a reference frequency. Any difference between the reference frequency and the oscillator's divided-down frequency results in the generation of a tuning voltage which is applied to the oscillator for reducing that frequency difference. When the reference frequency is equal to the oscillator's divided-down frequency, the oscillator is properly tuned.
To permit the frequency monitoring circuit to operate at relatively low frequencies rather than the higher frequencies associated with the oscillator, a so-called "prescaler" is disposed between the oscillator and the frequency monitoring circuit. The prescaler divides the oscillator frequency by a fixed number, 256 for example, and supplies the divided-down frequency to the frequency monitoring circuit. U.S. Pat. No. 3,980,958, assigned to the assignee of the present invention, discloses a tuning system employing such a prescaler.
One problem associated with the use of prescalers is that they are generally capable of operating properly only when the amplitude of their input from the oscillator is within a predetermined amplitude "window". Oscillator inputs to the prescaler which are either too large or too small and, hence, outside the prescaler's window, may be miscounted by the prescaler.
To illustrate this problem, it should be recognized that the outputs of VHF oscillators and UHF oscillators tend to vary in amplitude between minimum and maximum design limits for a given frequency. The amplitude of the oscillator outputs also varies as a function of frequency. Hence, it has been difficult to ensure that the amplitude of the oscillator outputs will be within the prescaler's window at all frequencies of interest.
In the past, this miscounting problem has been somewhat solved by adjusting the oscillator at the factory so that its output will remain substantially within the prescaler's window over the frequency range of interest. However, such factory adjustments tend to be time consuming and, therefore, expensive. Moreover, the oscillator may become misadjusted by subsequent handling of the receiver.
The problem noted above becomes particularly acute when the receiver is designed to receive multiband television signals, such as all VHF, UHF and CATV channels. For example, when the output of the VHF oscillator, used when CATV signals are received, sweeps from frequencies associated with the VHF band to frequencies associated with the CATV band, its output varies in amplitude considerably. In fact, the greatest fluctation in the output of the VHF oscillator usually occurs as it changes from VHF bands to CATV bands, and vice versa. Such amplitude fluctuations, when added to those normally encountered in a receiver not adapted to receiver CATV channels, increase the difficulty in reliably interfacing the oscillators with the prescaler. Hence, direct-access tuning has not been readily achievable in television receivers designed to receive a multi-band television signals, particularly those receivers designed to receive CATV channels in addition to all VHF and UHF channels.